I did a quick Google search and it looks like the A500 has USB 2.0 ports. You can use your USB 3.0 drive with them, but it will run at USB 2.0 speeds.
The good news is that it looks like your system should have an Express Card port. This means you can buy an Express Card to USB 3.0 adapter which will let your hard drive run at its full speed.
Here's an example of such an adapter: http://www.newegg.com/product/product.aspx?Item=N82E16839158037